The present invention relates to a method of the kind defined in the preamble of Claim 1 for treating wood-fibre pulp, particularly pulp that contains recycled paper.
The invention also relates to plant for carrying out the method.
Wood-fibre pulp of the kind with which the invention is concerned is treated typically in a number of stages which include dewatering of the pulp in a dewatering zone with the aid of a screw press, so as to increase pulp consistency from an input value of 4-6% to an output value of about 30%.
The pulp is then passed from the dewatering zone to a heating zone, through which the pulp is advanced while delivering steam thereto. The pulp is preferably shredded in this treatment zone, so as to enable the pulp to be heated with saturated steam to a desired temperature, both quickly and uniformly. On average, 300 kg of steam are consumed with each tonne of pulp treated.
The heated and finely-divided pulp is then passed to a disperser which enables the material to be treated mildly at high pulp consistencies. This is made possible by the mutually opposing toothed refiner discs, of which one rotates relative to the other. The disc clearance or gap can be adjusted during operation with the aid of an electro-hydraulic system.
The temperature of the treated pulp can vary from between 80xc2x0 C. and 120xc2x0 C. for instance, depending on the nature of the pulp being treated, among other things. There is a risk that pulp fibres will be damaged at excessively high temperatures. The pulp consistency may be as high as 30% when leaving the heating zone. Pulp consistency is reduced to some extent by the pressurized steam delivered to the pulp.
Water is normally added during the refining process to thin the pulp, and the consistency of the pulp pumped from the refiner will normally lie between 3.5 and 6%. Pulp consistency may, however, reach about 15% in some cases.
U.S. Pat. No. 5,176,793 (Kurtz) corresponding to SE-C 461 919 (Cellwood Machinery) teaches a method of the kind defined in the preamble of Claim 1 that enables the pulp to be given a high consistency by generating in the screw press a plug which effectively seals against steam leaving the system. It has been possible to achieve pulp consistencies of 30% or more by means of this method.
The sealing plug also enables the steam temperature to be increased in the heating zone, which has particular advantages when treating highly contaminated recycled paper. It has thus been possible to utilize temperatures of 120xc2x0 C. and higher. It has also been possible to increase steam pressure and therewith improve process yields.
These prior publications also describe a disperser for finely dividing dewatered and heated wood-fibre pulp. The disperser includes a rotor that carries a disc which has provided around a peripheral region teeth that coact with teeth on an opposing toothed ring or disc mounted in a surrounding housing, said ring or disc preferably being fixedly mounted and functioning to finely divide the pulp. The disperser also includes a pulp outlet. In some cases, the pulp outlet includes a connection with a screw press for advancing the pulp to an end of the press that is closed or sealed by valve means so as to form a sealing pulp-plug at said end, without separate departure of steam.
Although this known method and known plant provide very good results when treating recycled paper, they do not address the problem of bleaching recycled paper, which is often necessary. This applies both to the fibres present in the wood-fibre pulp and to the inks deriving from the print on the recycled paper. This problem is made more difficult by the fact that the bleaching chemicals used in the bleaching processes concerned require different conditions in order to achieve the best bleaching result, particularly with respect to the temperature of the pulp to be bleached. In addition, an increase in temperature, often to substantially above 100xc2x0 C.xe2x80x94which certain chemicals require for the best resultxe2x80x94also requires a corresponding increase in pressure.
U.S. Pat. No. 4,909,900 (Matzke, et al) teaches a bleaching method in which, subsequent to dispersion, the bleached wood-fibre pulp is delivered to a tank or container in which the bleaching method is carried out. The method thus lacks the step in which finely-divided pulp is delivered to a screw press or plug screw without separate steam departure. The object of this known method is to shorten the bleaching process to a duration of some few seconds.
WO 96/012062 (AGA) teaches a method which is characterized mainly by supplying oxygen to the process with the intention of limiting the amount of peroxide used. This known method also lacks the step of delivering the pulp to a screw press or plug screw downstream of the disperser, and the prevention of separate departure of steam.
Neither does this publication address the particular problem that is encountered when some of the bleaching chemicals relevant to the bleaching process react with oxygen and therewith lose their bleaching effect in the presence of oxygen.
An object of the present invention is to provide a method of the kind defined in the introduction that will make instaneous effective bleaching of the wood-fibre pulp possible to achieve.
Another object of the invention is to provide a method which will enable effective bleaching of pulp in conjunction with the treatment thereof, both with respect to the actual fibres in the pulp and also with respect to contaminants relating to printing inks present in the pulp.
A further object is to provide a method of the aforedescribed kind which includes a bleaching process and which is adapted to the different temperature conditions required by the various chemicals used, in order to obtain the best possible result.
Still another object of the invention is to provide a method which includes a wood-fibre pulp bleaching process and which functions effectively with the exclusion of atmospheric oxygen.
According to a second aspect, an object of the invention is to provide a treatment plant, particularly for the treatment of wood-fibre pulp that includes recycled paper, with which pulp can be bleached effectively in the absence of any essential auxiliary devices.
These and other objects are achieved with an inventive method that has the characteristic features set forth in the characterizing clause of Claim 1.
The most significant advantages afforded by the invention reside in the ability of bleaching wood-fibre pulp at temperatures higher than 100xc2x0 C., by providing a sealed, pressurized system that will prevent the ingress of air. This also overcomes the serious drawback that certain chemicals have a poorer bleaching effect in the presence of air, or may even be destroyed. The invention is also beneficial from an environmental aspect, since no chemicals are able to leak from the plant while the plant is operating.
The fact that the plug formed by the screw press effectively seals the system against the departure of steam from the system, while maintaining pulp consistency, enables the temperature of the system to be controlled effectively, such temperature control being essential to the effective bleaching effect achieved when applying the invention.
The steam temperature in the heating zone may reach 120xc2x0 C. or higher and is highly effective in relation to the addition of certain bleaching chemicals, which is advantageous when treating recycled paper that is highly contaminated with printing inks. Adjustment of the pressure of the steam delivered to the system can therewith contribute towards improving the yield of the treatment process.
Other bleaching chemicals require a lower temperature to achieve the best result. With this in mind, the invention enables the temperature to be reduced by supplying water of relatively low temperature to the system. This can be achieved by cooling the wood-fibre pulp with a comparatively large amount of low-temperature water in conjunction with the chemical supply, and thereafter reheating the pulp. This enables the thermal energy contained in the pulp immediately downstream of the disperser to be used in an optimal manner.
The pulp residence time in the reaction screw can be adjusted by varying the speed of the screw. It has been found in practice that in order for the bleaching chemicals used to give the best result, the pulp should have a screw residence time of longer than six minutes. A residence time of about ten minutes is usually the most effective residence time.
The bleaching chemicals may be charged to a shredder that is preceded by a preheater in the dispersing part of the plant and from there to the disperser itself and then to a screw press or to a plug screw downstream of the screw press.
The active phase of the bleaching process takes place thereafter in the bleaching section of the plant, which in one preferred embodiment of the invention commences with a shredder that disintegrates the pulp plug formed in the screw press or in the plug screw.
The major part of the bleaching process then takes place in the reaction screw where the pulp residence time is controlled with the aid of the speed at which the screw rotates, as mentioned earlier.
According to another aspect, the invention also relates to a wood-fibre pulp treatment plant, particularly wood-fibre pulp that contains recycled paper, the main characteristic features of said plant being set forth in Claim 4.
Other characteristic features of the invention and advantages afforded thereby will be apparent from the following description of a number of preferred embodiments of the invention. These embodiments are described with reference to the accompanying drawings, which illustrate the main components of an inventive treatment plant schematically and partly in section.